[fpc-pascal] Generics: constructor restriction fail

[Sven Barth]

On 31.10.2014 00:04, silvioprog wrote:
> Hello,
>
> Following this article:
>
> http://alex.ciobanu.org/?p=51
>
> The compiler does not check the constructor restriction.
>
> Try this test:
>
> {$mode delphi}
>
>    TTest = class
>    private // hidding the constructor to cause a compiler error
>      constructor Create;
>    end;
>
>    TGen<T: class, constructor> = class
>    end;
>
> constructor TTest.Create;
> begin
> end;
>
> var  Gen: TGen<TTest>;
> begin
>    Gen := TGen<TTest>.Create;
> end;
>
> It compiles well in FPC (from trunk), but the same code in XE is:
>
> "[dcc32 Error] Unit1.pas(36): E2513 Type parameter 'T' must have one
> public parameterless constructor named Create"
>
> It is a bug in FPC or I need to enable some directive switch?

"constructor" restrictions are not enforced currently, because with the 
way FPC handles generics compared to Delphi they are a bit useless. 
Consider this:

=== code begin ===

type
   TTest = class
   strict private // just to keep it hidden in the same unit
     constructor Create;
   end;

var
   t: TTest;
begin
   t := TTest.Create;
end.

=== code end ===

The above code snippet will compile, because the compiler will use the 
next best constructor that it can find which is at least the constructor 
of TObject.

Next snippet:

=== code begin ===

type
   TGeneric<T> = class
     procedure Test;
   end;

procedure TGeneric<T>.Test;
begin
   Writeln(T.Bar);
end;

begin
end.

=== code end ===

This will correctly compile in FPC, but not in Delphi, because FPC 
handles quite some type checking at specialization time not at 
declaration time. So this will fail to specialize (at compile time) if T 
does not implement a class method called "Bar".

Now let's put these two concepts together and exchange "T.Bar" with 
"T.Create" (and change the "Writeln" to something else of course ;) ) 
and it will still compile as long as T provides a Create method. Now as 
long T is a class this will always be the case, because there is at 
least the TObject constructor.

That's why the "constructor" constraint is currently not handled in 
anyway. That said there'd still be a usecase for this to ensure that 
there is a default constructor in the class. Also it might be worthwhile 
to rethink the type checking in case of mode Delphi for generics... :/

Regards,
Sven